Heyns, K. and L. Blazejewicz, Tetrahedron, Vol. 9 (1960) pp. 67-75, report the oxidation of primary aryl-substituted alcohols with air in the presence of platinum deposited on charcoal, or platinum dioxide. Heyns et al., supra, teach that the oxidation of primary aryl-substituted alcohols, such as benzyl alcohol, with oxygen in the presence of a platinum catalyst gives an aldehyde when the operation is carried out in a purely organic medium, while in an aqueous medium the oxidation proceeds as far as the corresponding carboxylic acid. Thus, in the case of benzyl alcohol, oxidation in an n-heptane medium gives benzaldehyde, while in an aqueous medium and in the presence of a little sodium hydroxide, benzoic acid is obtained almost quantitatively.
It is also known that hydroxybenzyl alcohols can be oxidized with oxygen in good yield to the corresponding hydroxybenzaldehyde if palladium is used as the catalyst (Marchand et al., U.S. Pat. No. 3,321,526 "Production of Hydroxybenzaldehydes", May 23, 1967).
The substantially identical catalytic activities of palladium and platinum have been demonstrated (Methoden der Organischen Chemie, 4th ed., Houben-Weyl, Vol. 4, Part 2, (1954) pp. 165-193; Dibella, E. P., U.S. Pat. No. 3,673,257, "Process for the Production of Aromatic Hydroxyaldehydes", June 27, 1972). The use of either of these noble metal catalysts results in rapid and almost complete absorption of oxygen. Palladium has been preferred because it selectively limits the oxidation to the aldehyde stage without any appreciable formation of the acid (Marchand et al., supra, column 1, lines 70-72).
In addition to the use of the noble metal catalyst it is observed that nearly all processes for the oxidation of hydroxy aryl-substituted alcohols to the corresponding aldehydes include the addition of caustic or a suitable alkaline agent. The caustic treatment eventually necessitates a neutralization step for the isolation of the desired product; this neutralization step also produces an undesirable brine waste stream.
It is desirable to produce hydroxy aryl-substituted alcohols efficiently and in good yield without the neutralization step that produces an undesirable waste stream. This can now be achieved by the process of this invention wherein aryl-substituted alcohols are oxidized in a multiphase system using an alkaline agent such as caustic in an aqueous solution, an organic liquid which is a solvent for the aldehyde formed, and a noble metal catalyst. This system permits the recycling of the caustic solution without neutralization and allows an efficient work-up and isolation of the desirable aldehyde product from the organic phase.